[mlir] Start splitting the `tensor` dialect out of `std`.

This starts by moving `std.extract_element` to `tensor.extract` (this
mirrors the naming of `vector.extract`).

Curiously, `std.extract_element` supposedly works on vectors as well,
and this patch removes that functionality. I would tend to do that in
separate patch, but I couldn't find any downstream users relying on
this, and the fact that we have `vector.extract` made it seem safe
enough to lump in here.

This also sets up the `tensor` dialect as a dependency of the `std`
dialect, as some ops that currently live in `std` depend on
`tensor.extract` via their canonicalization patterns.

Part of RFC: https://llvm.discourse.group/t/rfc-split-the-tensor-dialect-from-std/2347/2

Differential Revision: https://reviews.llvm.org/D92991